Use this method when you want to run a function in the reactor's thread
from another thread. Calling callFromThread
should wake up the main thread (where reactor.run() is
executing) and run the given callable in that thread.

If you're writing a multi-threaded application the callable
may need to be thread safe, but this method doesn't require it as such. If
you want to call a function in the next mainloop iteration, but you're in
the same thread, use callLater
with a delay of 0.